A flash memory is increasing its application to multimedia cards by its capability to retain information without power supply. A typical flash memory is illustrated in figures such as FIGS. 6 and 77 of JP 2001-196477 A, where integration is achieved by configuring multiple memory cells having matrix-aligned stacked gate electrodes overlying a semiconductor substrate via a gate insulating film.
As further described in the NAND flash memory disclosed in JP 2001-196477 A, select gate electrodes and floating gate electrodes are formed periodically in the memory cell region. Contact regions are formed on the surface layer of the semiconductor substrate, more specifically, beside the select and floating gate electrodes. Contact plugs are formed on the semiconductor substrate to allow application of voltage on the contact regions.
One exemplary approach in obtaining such configuration is forming contact plugs between a plurality of select gate electrodes which have been formed periodically along with floating gate electrodes over a semiconductor substrate via gate insulating film. The select gate and floating gate electrode structures, being formed at constant periodic spacing, provides lithography process margin. However, when the select gate electrodes and the floating gate electrodes must be formed in non-periodic spacing, lithography process margin cannot be obtained.
Increasing requirement for device integration and shrinking of design rules results in ever narrower gaps between the select gate electrodes and between the floating gate electrodes. Thus, it is becoming more and more difficult to obtain lithography process margin. Such conditions render dimension control of select gate electrode difficult, which may result in degradation in select gate electrode characteristics. The above described problems does not only concern flash memories provided with floating gate electrodes and/or select gate electrodes but also semiconductor devices with periodic formation of gate electrodes on the semiconductor substrate via gate insulating film.